GENEALOGY. 65 
The Third Subseries of Coroniceras begins with Oor. datum This species is 
remarkable for retaining, until a late stage of growth, the characteristics of the 
young of rotiforme, and for its exceptional form, the sides of the whorl being 
exceedingly divergent, the pile fold-like and heavily tuberculated, the abdomen 
gibbous and slightly elevated. 
There is one variety of rotiforme with very stout and gibbous whorls in the 
young, which cannot be distinguished from one variety of /atum until a late 
stage of growth, except by the singleness of the pile. Single pile occur, how- 
ever, in many specimens of all varieties of alum, so that this is not a distinction 
of constant value. Cor. datum must, therefore, be considered a direct descendant 
of rotiforme. From this species the transition to Cor,.Bucklandi? is accomplished 
by numerous intermediate forms. These exchanged the form of datum in the 
young for a sinemuriense-like stage, in which the abdomen became contracted 
in breadth, the sides parallel, and the channels deep. This stage was retained 
in some specimens for a long time, while in others it quickly gave place to 
the huge pil, parallel but gibbous sides, transversely broad whorls, and flattened 
abdomen of the adults of the typical Buckland. 
In other forms, with similar young, the pile assumed during their adult stage 
the usual aspect, with either only a trace of tubercles, or none. I have not been 
able to follow the transitions of this, or the sinemuriense variety, into the stout form 
of Buckland. There can be but little doubt, however, that the large form found 
at Lyme Regis differs only in having had a more accelerated development ; i. e. in 
skipping the double pile and large tubercles of the sinemuriense stage. It evi- 
dently acquired, at a very early period in the young, large untuberculated pile, 
and in old age was characterized by a very decided narrowing and rounding off 
of the abdomen, obsolescence and bending forward of the pilex, disappearance of 
the channels, and a broader and less elevated keel. 
The evidence of transition from the sinemuriense variety to Cor. orbiculatum 
rests upon similar grounds. The singleness and perfection of the pil are the 
only differences which separate the young of orbiculatum from the young of such 
forms as variety No. 5 of sinemuriense. In the adults, however, the narrowness 
of the abdomen, flatness of the sides, and their convergence outwardly, are 
marked differences in aspect, which were greatly increased by advancing age, 
The abdomen in some very large specimens became almost obtusely angular, as 
in Vermiceras, and the pile fold-like, much bent forwards, and the channels 
obsolete. 
Agassiceran Series. 
This series obliges us to return once more to Psioceras planorbe. It has two 
subseries. , 
First Subseries. —The young of Agas. levigatum® had a close resemblance to 
the young of the compressed varieties of Cor. kridion, and to Cor. rotiforme in some 
varieties, before the latter acquired tuberculated pile. But this likeness was 
1 Pl. iii. fig. 19-23; Summ. Pl. xii. fig. 16. * Pl, in, fig. 18; Summe-Pl xii ho. 17. 
8 Pl. viii. fig. 9-14; Summ. Pl. xiii, figs} 
